1. Field of the Invention
This invention relates to a meter and method of controlling decompression risks. More specifically the invention relates to a device and method whereby at least ambient pressure, breathing gas composition, and time are sensed or simulated, processed and the resulting information, or physical control signals, are sent to a display available to the person assuming a decompression risk or debit such as a diver, aviator, space traveler or other person moving from a high ambient pressure environment to a low pressure environment. In an alternative embodiment the signals are sent to a pressure control system to allow people to decompress at a predetermined level of safety.
2. Description of the Prior Art
Decompression sickness (DCS), sometimes called bends, is a hazard to people who are subject to a reduction in atmospheric pressure. That condition occurs in divers when they return to a pressure of one (1) atmosphere (ATA) from the deep portion of their dive, in pressurized caisson workers when they leave their pressurized job site, and in aviators and astronauts who are deliberately subjected to lower than normal atmospheric pressure.
Despite much research, the detailed causes of DCS remain unknown, although most experts feel that a central role is played by expanding gas bubbles. Bubbles are possible because inert gases such as atmospheric nitrogen dissolve in body tissues up to a limit determined by atmospheric pressure, and when the pressure is reduced, the prior dissolved gas is in excess of the new atmospheric limit, termed "gas supersaturation" and has a thermodynamic tendency to form bubbles.
Because a full valid theory is lacking, all methods to avoid DCS are empirical; that is, they are based on a knowledge of which methods, by experience, tend to make DCS less likely than other methods. Nearly all methods embody a mathematical simulation of the amount of gas thought to be in body tissues, and, again by mathematical simulation, seek to limit the amount of supersaturated gas.
Since the time of Boycott et al., 1908 [1], "safe" decompression procedures have nearly all been based on putting a maximum value on the allowable ratio of gas dissolved in tissues and ambient pressure, a so-called critical supersaturation ratio. Over the years, considerable refinement in the value of the critical supersaturation ratio has occurred. Many of the more modern decompression tables or schedules (collections of rules specifying pauses, or "stops" at intermediate pressures during return to a lower pressure after exposure to a higher one to allow inert gas to be safely excreted from body tissues) use a collection of critical supersaturation ratios that depend both on the diver's current depth and on the assumed inert gas excretion rates of multiple tissues. In these cases, the supersaturation ratio is expressed as a maximum allowable value above ambient pressure at each pressure, traditionally known as maximum permissible pressure values or "M-values". The 1956 U.S. Navy Decompression Tables developed by Workman, see Workman 1965 [2], and DCIEM (Canada) 1983 air diving tables, see Nishi et al. 1983 [14] are a particularly well-known embodiment of tables based on multiple critical supersaturation values.
Decompression tables can be restrictive in some situations since they are tabulated in specific increments of pressure and time. For example, decompression tables for divers typically use depth in 10-foot increments (33 feet of seawater (fsw) approximately equals 1 ATA), and time at depth at 10-min increments. In actual use a diver must choose a table that reflects the maximum depth ever attained during a dive (no matter how brief a time actually spent at that depth) for the entire time of the dive (the time from leaving the surface, 1 ATA, to beginning decompression). For example, a diver who was at 125 feet for 25 min would use the decompression schedule of 130 feet for 30 min even though the diver's pressure exposure was not that severe and the diver did not spend a full 30 min at the maximum depth. In a more restrictive example, a diver might have spent only 5 min at 125 feet depth, another 20 rain at 35 feet (called a multilevel dive), and still need to decompress very slowly according to the 130-feet/30-minute tabulated decompression schedule when it might have been safe for him to directly return to the surface without any decompression stops.
The usefulness of a decompression meter or computer that could sense the actual pressure exposure in real-time and tailor a decompression schedule for the user's individual pressure exposure is evident. An early embodiment of an analog mechanical device for this purpose was described in U.S. Pat. No. 3,457,393 to Stubbs and Kidd issued Jul. 22, 1969. Subsequent implementations using electrical and electronic components are described in U.S. Pat. No. 3,681,585 to Todd issued Aug. 1, 1972; U.S. Pat. No. 3,992,948 to D'Antonio et al. issued November 1976; U.S. Pat. No. 4,005,282 to Jennings issued January 1977; U.S. Pat. No. 4,054,783 to Seireg et al. issued October 1977; U.S. Pat. No. 4,109,140 to Etra issued August 1978; U.S. Pat. No. 4,188,825 to Farrar issued February 1980; U.S. Pat. No. 4,192,001 to Villa issued March 1980; U.S. Pat. No. 4,586,136 to Lewis issued April 1986; U.S. Pat. No. 4,658,358 to Leach et al. issued April 1987; U.S. Pat. No. 4,782,338 to Barshinger issued. November 1988; and U.S. Pat. No. 4,882,678 to Hollis et al. issued November 1989. None of these deviate importantly in concept from the approach in the Stubbs and Kidd disclosure.
The inventions described above are all based on algorithms that produce decompression schedules, which match or approximate a set of reference tabulated decompression schedules. When used for diving these are most often those officially promulgated by the U.S. Navy in 1956. An important caution is that the reference schedules were not designed for nor tested under conditions where the gauges and computers are seemingly most valuable, that is, multilevel pressure exposures. Tests (less than 600 total) supporting the 1956 U.S. Navy Decompression Tables for diving were performed very near the limiting conditions of maximum depth for full time as tabulated in the numerous Navy decompression tables.
A further problem known to those in the art is that no decompression schedule is perfectly safe (i.e. DCS never occurs) or perfectly unsafe (i.e. DCS always occurs). Every procedure has some finite probability of leading to DCS although the chance varies greatly from procedure to procedure. It is further known, and demonstrated convincingly by Gray, et al., 1947 [3], that the identical decompression procedure can cause DCS in some people while others are unaffected, and that a person suffering DCS on one occasion can often be free of DCS on a subsequent identical exposure. This extreme human variability further limits the confidence that should be placed on procedures or devices that can be traced to tests involving relatively small numbers of test pressure exposures under quite limited conditions.
Recent advances in the art have begun to address those limitations. In 1984, Weathersby et al. [4] demonstrated how simple decompression algorithms used for diving can be optimally and objectively tied to a body of test dive data. The tie is provided by application of the Principal of Maximum Likelihood, also well known to practitioners of statistics [5,6]. Using maximum likelihood, it is possible to find the set of algorithm parameters which best describes the observed incidence of DCS in the data set. In this way, the algorithm is "calibrated" against the data set. A necessary assumption for implementation of that method is that "safety" is not a binary condition of always or never DCS, but that any procedure has some finite probability of causing DCS. Thus the method comes closer to the actual decompression observations than had been possible in using critical supersaturation methodologies which presumed a sharp boundary between ill-defined "safe" or "unsafe" procedures.
Later, the methodology was extended into more complex pressure exposures that included more complex air diving profiles (Weathersby et al., 1985a [7]). This advance used a risk function (also known as hazard or survival function) to describe the instantaneous probability of DCS occurring and from the time integral of this function computed the overall probability of DCS occurrence. These functions and the methodology for computing probability of a failure or symptom occurrence are well known to practitioners of drug efficacy trials, and of industrial machinery reliability tests [5,6]. With a mathematical algorithm based on risk functions, now objectively calibrated, it was possible to construct new decompression tables at specified levels of acceptable risk (or probability of occurrence) of DCS (Weathersby et at., 1985b [8]. This allows the user to compute decompression schedules at a level of risk to suit his particular needs, an option not previously available.
Several problems remained with probabilistic algorithms prior to the present invention. First, although overall decompression schedules could be produced at a specified risk level, information on time-course events was missing. This meant that while the overall incidence of DCS could be reliably described, times of high and low risk could not be viewed with the same degree of reliability. As will be presented further below, algorithms could only be calibrated to provide a time resolution of DCS risk of about one day when in practice it was desired to provide decompression recommendations in near real-time or at least in time scales of minutes or hours. It was necessary to find a means of objectively calibrating the probabilistic decompression algorithm with DCS information on a finer time scale. Otherwise, such an algorithm could be seriously misleading and lead inadvertently to a higher, or lower, level of risk than was deemed acceptable when used in real time or for new dives beginning less than 24 hours after a preceding dive.
Also there was no practical fast way to optimize a decompression schedule, i.e., find the one with the shortest decompression time arising from a probabilistic algorithm. As discussed in Weathersby et al., 1985b [8]), many decompression procedures are possible after a given pressure exposure, all of which may have a similar chance of causing DCS. An optimal schedule is defined as the fastest overall means of arranging decompression stop depths and times to minimize total time used for going from higher to lower pressure while still remaining below the acceptable DCS risk level. Weathersby et al. [8] shows an example where over 10,000 plausible decompression schedules could be used at the end of a moderately severe exposure each producing the same level of risk. By this invention, we have developed a means of obtaining an optimal, or near optimal, schedule with substantially less computational requirements than would be involved in evaluating thousands of possibilities.